Tree diversity effects on leaf insect damage on pedunculate oak: The role of landscape context and forest stratum

Abstract

Tree diversity has long been recognized as a major driver of insect herbivory in forest ecosystems. However, predicting the strength and direction of tree diversity effects in real-world situations has proven elusive. One likely reason is that most studies have focused on within-stand dynamics and insufficiently captured other ecological drivers of insect herbivory that can act at broader (i.e., landscape) and finer (i.e., individual trees) scales. We measured herbivory as leaf area consumed by insect herbivores in pedunculate oaks (Quercus robur) growing in mixed and pure forest stands in southwestern France. We assessed the effects of oak spatial isolation within the landscape, tree stand diversity, forest canopy stratification as well as the influence of leaf traits on insect herbivory. Insect herbivory increased with stand isolation regardless of tree diversity. Diversity effects were contingent upon the canopy stratum as insect herbivory in mixed stands exceeded that of pure stands only in the upper stratum. Leaf traits varied between pure and mixed stands and among canopy strata. Insect herbivory was negatively correlated with LDMC and positively with SLA. However, the observed effects of tree diversity, canopy stratum and stand isolation on insect herbivory were only partially driven by variability in oak leaf traits. Our findings illustrate that, in real-world contexts, insect herbivory can be driven by a complex interplay of multiple, scale-dependent drivers. They help step forward towards a more profound understanding of the complex forces drive insect herbivory in managed forest ecosystems.